1. Field of the Invention
The present invention relates generally to a system for aligning a device relative to a reference line and, more particularly, to a method and apparatus for acquiring a two- or three-dimensional frame of reference relative to the line, and relative to the device, and for then transferring the reference frames to determine and adjust the orientation of the device with respect to the line.
2. Description of the Prior Art
In order to control equipment such as sensors, guns, cameras and antennae mounted on vehicles such as aircraft or spacecraft, it is important to align the equipment boresights with respect to a reference axis on the vehicle. The Armament Datum Line (ADL) is recognized to be a standard reference line, and a number of methods exist for bringing weapon or navigational stations into alignment with the ADL on a variety of aircraft. The ADL defines the center line of the aircraft; however, it is more than simply a line because it also provides a roll reference. Although reference is made to an ADL for alignment applications involving spacecraft and aircraft, the present invention is also useful in oil drilling, civil engineering, construction and medical applications, among others, which involve the alignment of any device with respect to a structural or virtual reference line.
One alignment method using the ADL of an aircraft, as shown in FIG. 1, involves attaching two brackets or adapters 20 and 22 to an aircraft 24 at two respective locations along the ADL 26. In addition, each station on the aircraft is fitted with its own adapter (not shown). A telescope 28 is then installed in the leading or forward end 22 bracket and is used to align with the rear or aft end bracket. With reference to FIG. 2, a target board 30 is set at a precise distance from the telescope 28. The target board is aligned so that a reticle 32 from the telescope falls upon a ADL fiducial 30 on the target board. The telescope is then moved from station adapter to station adapter while each station is boresighted with its own fiducial 36 on the target board. It should be apparent that the use of the telescope and target board is limited to the transfer of parallel lines (ADL to other station lines) to thereby align the stations.
In the second alignment method, a "Christmas Tree" adapter 40 is attached to the aircraft (see FIG. 3) and is aligned to the ADL. Additional adapters (not shown) are also provided on each station and a telescope 42 is positioned at various points 44, 46 and 48 around the tree to align each station. In order to accommodate all the stations on an aircraft, this tree is necessarily large and onerous. Again, this method of alignment is limited to the transfer of parallel lines.
Both of these methods for boresight alignment have procedural and equipment aspects which seriously limit their ultimate accuracy. Some of these limitations include: the reliance on the proper alignment of the human eye with the optical system (parallax) for error readings; the correct positioning of the target board not only in standoff position but in pitch, yaw and roll positions; the use of a finite focal length reticle as a reference; the movement of the target board during alignment on the flightline due to wind and other factors; the warping or bending of the Christmas tree; and the movement of the aircraft itself, among other limitations.
Beyond accuracy, there are two other factors which make these methodologies undesirable: the size and weight of the auxiliary equipment, and the time needed to complete a station alignment. For example, the mounting stand 50 (FIG. 2) for a target board is 10 feet tall and weighs approximately 500 pounds. The alignment procedure for an aircraft using the target board requires the elevation of the front of the aircraft to relieve weight on the nose wheel using a 600 pound jack. The station adapters themselves typically weight 25 to 35 pounds and are awkward. The alignment procedure for the Apache helicopter typically involves removal of the windshield in order to install the "Christmas Tree" alignment adapter for a heads-up display.
The two boresighting methods discussed above employ optics to acquire the reference axis. A number of boresighting systems exist which employ gyroscopes to align a device with respect to another device. For example, U.S. Pat. No. 4,012,989 to Hunt et al. discloses an inertial sighting system for slewing the axis of a device which is mounted on an aircraft. The disclosed system comprises a pair of gyroscopes and a hand-held sighting device, which also comprises a pair of gyroscopes. Both sets of gyroscopes are initially caged to align the spin axis on each gyroscope on the aircraft mounted device with the spin axis of a corresponding one of the gyroscopes on the hand-held device to establish an arbitrary reference system between the two devices. Once the gyroscopes are uncaged on the sighting device, data is continuously fed from the hand-held device to generate orientation command signals for a gun.
U.S. Pat. No. 3,731,543 to Gates discloses a gyroscopic boresight alignment system comprising a master sensor unit having two gyroscopes which is mounted on an aircraft with respect to its armament data line. The system also comprises a remote sensor unit having a single gyroscope which is mounted on equipment. The misalignment of equipment is determined by comparing angular rates of the aircraft and equipment axes with respect to a parallel relationship with the ADL.
U.S. Pat. No. 3,930,317 to Johnston discloses an electronic azimuth transfer system comprising a navigator which is mounted on a vehicle. A remote sensor coupled to the navigator aligns itself with respect to North as does the navigator. The remote sensor is thereafter moved to a gun or other equipment to indicate equipment alignment with respect to North.
Prior gyroscopic alignment systems such as those discussed in the above-referenced patents are disadvantageous for several reasons. They are limited in operation to transfer only parallel lines with respect to a reference line, i.e., the ADL. Further, the systems in the Johnston and Gates patents do not provide for 3-axis detection. As a result, the accuracy of these systems is limited by the manner in which the gyroscopes on the master and slave inertial sensors are oriented with respect to each other. Specifically, if the hand held sensor is inadvertently rotated around the spin axis of the single gyro, the gyro senses no motion. Thus, the other two axes will no longer align with the axes of the double-gyro unit. This will cause a "cross coupling" error in the information produced by the device.